Minimum number of samples for the Branched Path Tracing
Cycles generates each sample type as different "images" which are then combined into the final render. Each of these passes can be made available in the render layer settings which can then be used within the compositor.

While cycles could be made to work with one or more of these passes being missing, there is no benefit to doing it. A simple scene with one sample of each pass can be rendered in less than a second. It is only when you are rendering with hundreds or thousands of samples that a pass adds any time to the render, having one sample of a type you don't use can only add less than a second to your render and the couple of MB that could be saved in memory use will not make a difference on any recent computer.

AA is Anti-Aliasing, it softens the edges to hide the jagged edges seen from sudden colour changes. 64 AA samples is high, the AA samples defines how many neighbouring pixels are used to determine the final pixel colour and as you noticed it multiplies the final sample count.

If you want to know the effect of each pass then enable them all in the render layers and mix them in the compositor where you can turn each on/off to see how they effect the final image. CGCookie has a fundamentals of compositing series that explains how to do this.